An electric power steering apparatus which performs an assist control of a steering apparatus of a vehicle by means of rotational torque of a motor, applies driving force of the motor as an assist load to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate an assist torque (steering assist torque), such a conventional electric power steering apparatus performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a current command value and a detected motor current value becomes small, and the adjustment of the voltage supplied to the motor is generally performed by an adjustment of a duty ratio of a PWM (Pulse Width Modulation) control.
A general configuration of a conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft or a steering wheel shaft) 2 connected to a steering wheel 1 is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack and pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. Further, the column shaft 2 is provided with a torque sensor 10 for the detecting steering torque of the steering wheel 1, and a motor 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. Further, the column shaft 2 is provided with a steering angle sensor 15 for detecting a steering angle, and the steering angle θ detected by the steering angle sensor 15 is inputted into the control unit 30. The control unit 30 calculates a current command value of an assist (steering assist) command based on the steering torque Tr detected by the torque sensor 10 and a vehicle speed Vel detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 based on a voltage control value E obtained by performing compensation and so on with respect to the calculated current command value. Moreover, it is also possible to receive the vehicle speed Vel from a CAN (Controller Area Network) and so on.
The control unit 30 mainly comprises a CPU (including an MPU and an MCU), and general functions performed by programs within the CPU are shown in FIG. 2.
Functions and operations of the control unit 30 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque Tr detected by the torque sensor 10 and the vehicle speed Vel detected by the vehicle speed sensor 12 are inputted into a current command value calculating section 31 that calculates the current command value Iref1. The current command value calculating section 31 calculates the current command value Iref1, which is a control target value of a current supplied to the motor 20, based on the inputted steering torque Tr and the inputted vehicle speed Vel by means of an assist map and so on. The current command value calculating section 31 may further comprise a phase compensating section for improving a stability of a steering system, a center-responsibility improving section that performs a maintenance of the stability in a dead zone of an assist characteristic and a compensation of static friction, a robust stabilization compensating section that eliminates a resonance frequency peak of a resonator system comprising an inertial element and a spring element included in the detected torque and compensates a phase shift of a resonance frequency to inhibit the stability and the responsibility of a control system, and so on.
The current command value Iref1 is inputted into a current limiting section 33 through an adding section 32A, a current command value Iref3 of which a maximum current is limited is inputted into a subtracting section 32B, a deviation Iref4 (=Iref3−Im) between the current command value Iref3 and a motor current value Im that is fed back is calculated, and the deviation Iref4 is inputted into a PI control section 35 that is a current control section for improving an characteristic of a steering action. The voltage control value E of which a characteristic is improved by the PI control section 35 is inputted into a PWM control section 36, moreover, a motor 20 is PWM-driven through an inverter 37 as a driving section. The current value Im of the motor 20 is detected by a motor current detector 38, and is fed back to the subtracting section 32B. FETs are used as driving elements within the inverter 37, which comprises a bridge circuit of FETs. Moreover, the current limiting section 33 is not essential.
Further, a compensated signal CM from a compensating section 34 is added to the adding section 32A, which performs compensation of a system by adding the compensated signal CM and improves convergence, an inertial characteristic and so on. The compensating section 34 adds a self aligning torque (SAT) 343 and the inertia 342 by an adding section 344, moreover, adds the convergence 341 to an addition result by an adding section 345, and makes the compensated signal CM of the addition result of the adding section 345.
In such the electric power steering apparatus, the action is obstructed by the friction of the reduction gears and the rack-and-pinion mechanism for transmitting the assist force, the steering wheel is not returned to the neutral point, and the vehicle may be hard to become the going straight state, although the vehicle is in such a running state as to be wanted to return to the going straight state.
As a prior art for controlling an assist characteristic at the time of returning the steering wheel, there is a steering wheel return control disclosed in the publication of Japanese Patent No. 4407935 B2 (Patent Document 1). A control structure disclosed in Patent Document 1 calculates a target steering speed from the steering angle, the vehicle speed and the steering torque, and calculates a compensation current depending on a deviation between the actual steering speed and the target steering speed.